While saving file this message comes
So, How to find out these uncompatible objects?
You would need an install of your software WITHOUT proxy objects (object enablers installed)
Then you could open the file and see (actually NOT see) the proxy objects for yourself and decide how to deal with them, or not.
It just looks like another form of 'encouragement' to upgrade your product to the latest version.
Because it becomes increasingly difficult to work in your version with data from newer versions.